Method eliminating discontinuities in a quartz article

ABSTRACT

The invention relates to a method of elimination discontinuities in a quartz tube by means of a thermal treatment. According to the invention a plasma burner is used for this treatment in which the quartz tube is passed through an electrical coil being part of this plasma burner, while an inert gas is direct to the quartz tube. As a result the quartz tube is locally heated while mechanical discontinuities can disappear and possible impurities can be removed from the quartz. The part of the quartz tube in which the impurities have accumulated is removed later, if necessary.

United States Patent Spiessens 1 Mar. 1l, 1975 METHOD ELIMINATING 3,156,549 11/1924 Kelemen 65/32 DI ONTINUITIES IN A UARTZ ARTICLE 3,171,714 3/19 Jones et al.... 65 DIG. 4 SC 3,730,696 5/1973 Pointu et al. 1 65/30 [75] Inventor: Raymond rancms Sme se s, 3,741,739 6/1973 Baker 65/120 Emmasingel, Eindhoven, 3,764,286 10/1973 Antczak et a1.... 65/D1g. 8 Netherlands 3,788,827 1/1974 DeLuca 65/30 [73] Assign: gf g g Corporanon New Primary Examiner-S. Leon Bashore Assistant Examiner-Frank W. Miga [22] Filed: Oct. 25, 1973 Attorney, Agent, or FirmFrank R, Trifari [2]] App]. No.1 409,419 7 [57] ABSTRACT [30] Foreign Application Priority Data The invention relates to a method of elimination dis- 7 continuities in a quartz tube by means of a thermal Nov. 1972 Netherlands 7214796 treatment 52 us. c1 65/32 65/86 65/111 Acwding the inv'emkm a P1551a burner is used 65ll20 4 this treatment in which the quartz tube is passed [51] Int. C03c /00 CO33 15/141303) 21/00 through an electrical coil being part of th1s plasma [58] Field of Search 65 H 32 DIG 4 burner, while an inert gas is direct to .the quartz tube. 86 As a result the quartz tube is locally heated while mechanical discontinuities can disappear and possible [56] References Cited impurities can be removed from the quartz. The part of the quartz tube in which the impurities have I 969 658 :A I I P PATENTS 65H 11 X accumulated is removed later, if necessary.

, c vame 2,897,126 7/1959 George 65/111 x 7 Clams, 2 Drawlng Figures PATENTEB HAR] 1 I975 sum 1 BF 2 Fig.1

HJENTED M I 1 I913 Fig. 2

METHOD ELIMINATING DISCONTINUITIES IN A QUARTZ ARTICLE The invention relates to a method of eliminating discontinuities, by means of a thermal treatment, in an elongated quartz article in which a relative movement of the article with respect to a concentrated thermal source is realized, said movement being effected in a direction which is substantially parallel to the longitudinal axis of the said article, a gas required for the thermal source being passed to the article in such a manner that said gas is at least present at the area of said thermal source near the article, the article being locally brought to above its softening point. The invention also relates to a device for performing the said method and to a quartz article manufactured by said method.

A known method of the kind mentioned above is described, for example, in the first addition 32970 of French Patent specification No. 588260. In this French patent of addition is described how the surface of an elongated quartz article is smoothed. The elimination of discontinuities in this case thus means the elimination of rough spots on this surface. A drawback of this known method is that the thermal source mainly heats only one side of the quartz article. For smoothing the quartz surface a rotation of this article is then required. The said French patent of addition also states that the thermal source may be an electrical are. It is not further described where this are is present. Neither has it been described that this are might be around a cross-section of the article. A general drawback of the method according to said French patent of addition is therefore that for simultaneous uniform heating of the outer wall of a disc of the quartz article two or more simultaneously operating concentrated thermal sources are necessary and/or the article must be rotated. An internal thermal treatment in the case of a tubular article is not at all mentioned in this patent of addition.

An object of the invention is to obviate or at least to mitigate the said drawback.

According to the invention a method of eliminating discontinuities, by means of a thermal treatment, is an elongated quartz article in which a relative movement of the article with respect to a concentrated thermal source is realized, which movement is effected in a direction substantially parallel to the longitudinal axis of said article, and whereby a gas required for the thermal source is passed to the article in such manner that said gas is at least present at the area of said thermal source near the article, and in which the article is locally brought to above its softening point is characterized in that the relative movement of the article is a movement with respect to an electrical coil surrounding said article and whereby the electrical coil is part of the thermal source which is formed as a plasma burner.

A plasma burner is understood to mean in this case a discharge device in which a plasma having a high gas temperature is generated in a gas by means of a high frequency alternating field.

In this connection discontinuities are understood to mean grooves and other irregularities in the surface of the quartz article as well as gas inclusions; also discontinuities may be understood to mean impurities in the quartz, for example, the presence of water and/or nonvolatile impurities (such as, for example, iron or copper) in this quartz article.

An advantage ofa method according to the invention is that the article need not be rotated during the method. In addition, in this methodheating on all sides of the article is substantially equally intensive due to the use of the plasma burner.

The gas may be passed, for example, along the outer side of the quartz article. The coil is then'connected to a voltage source of high frequency and a plasma discharge is generated in the gas which if necessary is first ignited, for example, with the aid of an auxiliary electrode.

In an advantageous method according to the invention in which the article has the shape ofa tube the gas is passed through the tube and the plasmadischarge is effected in the tube. A

An advantage of this preferred embodiment is that in the plasma the regions having the highest'temperature are present along the quartz wall.

In a further preferred method according to the invention in which the article has the shape ofa tube whose internal diameter is less than five mm the gas is at least present between the tube and the electrical coil.

An advantage of this method may be described, for example, with reference to exhaust tubes of quartz. When such tubes are provided with external grooves this may give rise to leakage if these tubes are connected by means of rubber seals to a vacuum system. The external tube walls may be made very smooth with the aid of the latter preferred method so that the said leakages will no longer occur.

In a further preferred embodiment of a method according to the invention in which the article has the shape of a tube whose length is longer than that of a product to be made from said tube, the portion of the tube which in the operating condition of the plasma burner passes last through the electrical coil is removed at a later stage and thus does not form part of the product to be made from the tube.

An advantage of the latter preferred embodiment is .that impurities which are present in the quartz are that the impurities in the part of the quartz tube which will be used have decreased.

In a further special embodiment according to'the invention for processing tubular articles this method has been used as part of a quartz tube drawing process in which the gas for the plasma discharge isappliecl in a quartz furnace to the quartz tube, and in'which'this tube is directly transported from the furnaceto the plasma burner.

An advantage of this method is that aquartz tube may be directly obtained during the quartz'tube drawing process, which tube may befree fromstripes and free from impurites to a great extent.

In an improvement of the latter method the'pl'asma burner is adjusted for a smaller thermal development after drawing of a quartz tube part and subsequently the drawing process is carried out for a relatively short time using said decreased thermal development of the plasma burner, whereafter the-plasma burner is adjusted to the larger thermal developmentagain and the drawing process is continued.

An advantage of this improvement is that'the impurities are already eliminated during the drawing process because they remain in relatively short quartz tubes which are drawn during the decreased thermal'development of the plasma burner.

It is feasible that the impure quartz tube portions later constitute the pinch parts of a quartz discharge tube.

According to a further preferred method the short quartz tubes which are drawn during the decreased thermal development are separated from the other quartz tube parts during further processing of the quartz. These short quartz tubes may then be destroyed. r

In a device for performing a method according to the invention including a thermal source, formed as a plasma burner, provided with an electrical coil and in which a gas supply is present the intensity of either the electrical supply of the coil and/or the gas supply can be regulated.

An advantage thereof is that the intensity of the thermal development ofthc plasma burner can be varied at suitable instants so that a separation can be made between the quartz tubes free from impurities and the quartz tube parts containing the impurities.

The plasma burner may be either completely extinguished or it may burn at a low intensity.

The invention will be further described with reference to a drawing in which:

FIG. 1 is a diagrammatical view of a device for performing a method according to the invention with a quartz furnace and a plasma burner for eliminating discontinuities in a drawn quartz tube.

FIG. 2 is a diagrammatical view of a second device for performing a method according to the invention likewise provided with a plasma burner which mainly serves for eliminating grooves in the outer surface of quartz tubes. for example, exhaust tubes.

In FIG. 1, 1 is a vessel of a furnace which is heated in a manner not further shown. 2 is a liquid mass of quartz present in said furnace. Quartz is introduced in a solid state into the furnace through a supply duct 3. solidifying quartz flows out at an aperture 4. This quartz flow which has a tubular form is denoted by 5. 7 is a reservoir of argon gas. This argon is passed through a tap 8 to the furnace I through a duct 9. This argon gas flows through a connection part 10 to the in-' ternal part of the quartz tube 5. An electrical coil 11 which surrounds the quartz tube 5 is present slightly below the furnace 1. This coil 11 is fed from a high frequency generator 12 connected to two supply terminals 13 and 14. The high-frequency generator 12 is controllable.

The method to be performed with this device for obtaining stripe-free clear quartz is as follows. The quartz flows at aperture 4 from the furnace and the quartz tube 5 produced there moves at a regular rate through the coil 11. Since there is a high-frequency supply across this coil 11 and a discharge in the argon gas at the area ofthis coil is initiated by an ignition device not further shown, quartz tube 5 is internally heated at the height of the coil 11. Possible gas inclusions in the wall ofthe tube 5 disappear because they come in free communication with the interior of the quartz tube 5. In addition it is ensured by this heat treatment that possible impurities in the quartz are displaced in such a manner that they are concentrated near the last heated part of the quartz tube. After such a length of quartz has been drawn in this manner that the concentration of the impurities near the last drawn part'becomes undesirably high, the high frequency generator 12 is adjusted in a different manner such that the intensity of the plasma discharge in the coil 11 is greatly reduced and subsequently a length of at least once the height of the electrical coil is drawn. The generator 12 is then adjusted to its original intensity and the drawing process is continued. In a following part of the process the piece of the quartz tube 5 drawn with the reduced Intensity of the plasma burner, thus with the reduced intensity of the supply of the coil 11, is cut loose from the tube and destroyed, for this is the part in which the impurities of the other part of the quartz tube have accumulated. An advantage of this device thus is that quartz can be obtained which is substantially free from stripes and is very clear. The auxiliary device 12 and the plasma burner 11 may be added, for example, to an ordinary quartz furnace 1.

FIG. 2 shows a quartz tube 30 whose internal diameter is rather small, namely for example 3 mm. This tube serves at a later stage, for example, as an exhaust tube for exhausting an incandescent lamp during manufacture or for evacuation and subsequent gas-filling of a discharge tube. 31 denotes an electrical coil which is again connected to a high-frequency generator 32. 33 denotes a cooling cylinder to be cooled with liquid. 34 and 35 are two nozzles for the supply of gas and 36 and 37 are two pipes near the upper side of the tube 30 which serve to conduct away the gas again. namely in the direction of the arrows. In the gas which is introduced through the nozzles 34 and 35 between the tube 30 and the cooling cylinder 33 a plasma discharge developing heat is brought about by a high-frequency supply of the coil 31. The tube 30 is moved along the axis of the coil and this in a direction which is substantially parallel to the longitudinal axis of this tube, and the outer surface ofthis tube is heated in the described way so that possible grooves present in this surface are eliminated.

An advantage thereof is that when this tube is used as an exhaust tube, possible leakages between the tube and the exhaust mechanism are substantially eliminated. I

What is claimed is:

I. A method of eliminating discontinuities and removing impurities by means of a thermal treatment, in an elongated quartz tube comprising: first providing a plasma burner thermal source which includes a coil and which generates a high frequency field; second, moving the tube with respect to said thermal source in a direction which is substantially parallel to the axis of the tube, third, passing an inert gas to the burner thermal source and to the tube in such a manner that said inert gas is at least present at the area of said burner thermal source near the tube to bring the tube locally to a temperature above its softening point.

2. A method as claimed in claim 1 in which said third. passing step includes the discharge of said gas into said tube, causing a plasma discharge in the tube.

3. A method as claimed in claim 1 in which the tube has an internal diameter of less than 5 mms. and said third, passing step includes directing said gas between the tube and the coil.

4. A method as claimed in claim 1 further including a fourth step of severing the axial section which forms one end of said tube and which end last passes through the electrical coil.

5. A method as claimed in claim 1 further including an initial step before said first providing step ofdrawing a tube from a quartz furnace and said third. passing LII first providing step of sequentially adjusting said plasma burner to said reduced thermal output, drawing an axial section at least as long as the axial length of said coil and adjusting said plasma burner to said given thermal output.

7. A method as claimed in claim 6 further including the step of severing said axial section which is at least as long as the axial length of said coil immediately before said second, moving step. 

1. A method of eliminating discontinuities and removing impurities by means of a thermal treatment, in an elongated quartz tube comprising: first providing a plasma burner thermal source which includes a coil and which generates a high frequency field; second, moving the tube with respect to said thermal source in a direction which is substantially parallel to the axis of the tube, third, passing an inert gas to the burner thermal source and to the tube in such a manner that said inert gas is at least present at the area of said burner thermal source near the tube to bring the tube locally to a temperature above its softening point.
 1. A method of eliminating discontinuities and removing impurities by means of a thermal treatment, in an elongated quartz tube comprising: first providing a plasma burner thermal source which includes a coil and which generates a high frequency field; second, moving the tube with respect to said thermal source in a direction which is substantially parallel to the axis of the tube, third, passing an inert gas to the burner thermal source and to the tube in such a manner that said inert gas is at least present at the area of said burner thermal source near the tube to bring the tube locally to a temperature above its softening point.
 2. A method as claimed in claim 1 in which said third, passing step includes the discharge of said gas into said tube, causing a plasma discharge in the tube.
 3. A method as claimed in claim 1 in which the tube has an internal diameter of less than 5 mms. and said third, passing step includes directing said gas between the tube and the coil.
 4. A method as claimed in claim 1 further including a fourth step of severing the axial section which forms one end of said tube and which end last passes through the electrical coil.
 5. A method as claimed in claim 1 further including an initial step before said first providing step of drawing a tube from a quartz furnace and said third, passing step includes passing the inert gas for the plasma discharge through said quartz furnace to the axial bore of said drawn quartz tube, said moving step including moving said tube directly from the furnace to the plasma burner thermal source, said plasma burner thermal source having a given thermal output.
 6. A method as claimed in claim 5 wherein the output of said plasma burner thermal source has both said given thermal output and a reduced thermal output, and further including the steps immediately after said first providing step of sequentially adjusting said plasma burner to said reduced thermal output, drawing an axial section at least as long as the axial length of said coil and adjusting said plasma burner to said given thermal output. 